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Derivative Financial Instruments (Narrative) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Derivative Financial Instruments [Abstract]      
Accumulated other comprehensive loss $ (2,172) $ (11,064)  
Gain or loss from discontinuing cash flow hedge treatment 0 0 $ 0
Energy trading contracts, gain (loss) $ 3,000 $ 12,300 $ 23,100